1. Field of the Invention
This invention is related to transient voltage surge suppression circuits and, more particularly, to a circuit arrangement for voltage surge suppression which provides protection against voltage surges and against failure of the components of the suppression circuit.
2. Description of the Prior Art
It has been determined that transient voltage surge suppressors are needed to protect consumer electronic equipment now commonly found in consumer households. Products such as microwave ovens and personal computers are susceptible to very short duration transient voltage spikes. A characterization of these spikes is provided by the Institute of Electrical and Electronic Engineers, IEEE Standard 587-1980. This standard describes voltage surges with durations of typically about 28 microseconds, peak voltages of about 6000 volts and peak follow current of about 3000 amperes. Even though the surges are of very short duration a cumulative effect occurs and causes premature appliance failure. One method of suppressing these surges is to use metal oxide varistors, such as are used in the voltage spike protectors sold by General Electric Company under its registered trademark GE-MOV.RTM., in combination with a current tap to limit or clamp these surges to safe operating levels. The energy of the surge is absorbed by the metal oxide varistor (MOV) instead of the attached appliance. This arrangement works well until the MOV reaches its end of life. The primary failure mechanism of an MOV is to go into a low resistance mode causing burning and smoking of the varistor, which can be alarming and disruptive to the consumer. A significant improvement would be achieved by providing a reliable protection means that allows the MOV to clamp surges effectively and also sense when the MOV end of life is approaching and remove power from the MOV prior to any failure of the MOV.
In a common household outlet as shown schematically in FIG. 1a the receptacle has three conductors: line 10, neutral 12 and ground 14. Surges may be generated from line to neutral, line to ground or neutral to ground. It is therefore desirable to provide a product to protect all three possible surge modes. Prior art surge suppressors utilize an MOV 16 connected from line to neutral as shown in FIG. 1a, but provide no protection of the MOV itself or indication to the user that the MOV is nearing its end of life. An alternative prior art surge suppressor uses the arrangement shown in FIG. 1b with a current limiting fuse or a temperature sensitive switch 18 to disconnect the MOV 16 from the power source. This arrangement was unsatisfactory from a surge suppression standpoint, because surges from either line to ground or neutral to ground are not suppressed, and thus the equipment is still vulnerable to certain types of surges. The prior art addressed this problem by the use of three MOV's, one 20 connected from line to neutral, one 22 connected from line to ground and one 24 connected from ground to neutral as shown in FIG. 2a. None of the MOV's as shown in FIG. 2a are protected from current or temperature variations beyond the normal operating condition. An alternative circuit is shown in FIG. 2b in which a current limiting fuse or switch 26 is connected in series with the line to neutral MOV 20 with the other two MOV's 22 and 24 having no similar protection.
Another alternative prior art arrangement is shown in FIG. 2c in which a current limiting fuse or switch 28 is connected in series with the MOV 22 connected between line 10 and ground 14 with no other protection used for the line to neutral MOV 20 or neutral to ground MOV 24. Yet another prior art arrangement is illustrated in FIG. 2d in which one MOV 22 is connected from line to ground and a second MOV 24 is connected from neutral to ground. If the MOV 22 connected from line to ground should fail short a direct short from line to ground is created. This may create a substantial hazard to the user if the user's appliance has a defective or unconnected ground, because the housing of the appliance would then receive full line voltage. In each of the embodiments shown in FIGS. 1 and 2 no mechanism is provided to protect against end of life failure of any of the MOV's. The user has no indication that a particular MOV was about to fail or that the circuit protection was no longer reliable.